In general, for a line of energy storage cells connected in series, for example, each cell comprising a supercondenser, some differences in charging voltage may arise between the cells due to:                differences between the capacitance values inherent in the manufacture of the cells,        the existence of leakage currents of different intensities.        
The persistence of these discrepancies over time can cause accelerated ageing, or even the destruction, of the cells which endure the highest charging voltages.
Different methods have been proposed to correct these imbalances.
For example, US patent application 2003/0214267 discloses a system for energy storage consisting of a line of cells, for example capacitive cells, assembled in series. This system comprises, for each cell, a balancing circuit to balance the leakage currents associated with the cells. This balancing circuit is formed by one resistor and one diode. This application, US 2003/0214267, also describes a balancing circuit incorporating an operational amplifier.
Also known, from U.S. Pat. No. 5,659,237, is a device for balancing the charge within a line of energy storage cells connected in series. This device includes a transformer of the flyback type making it possible, in each switching cycle of a MOSFET transistor, to transfer a certain quantity of energy to those cells in the line which exhibit the lowest voltages. This device draws energy to be transferred from a charger which is itself connected to an AC source.
US patent application 2005/0269988 describes an energy storage device enabling balancing on two levels, to with, intra-modular and inter-modular.
U.S. Pat. No. 6,771,045 describes a device for charging with power and balancing the packs of batteries using a processor.
U.S. Pat. No. 6,150,795 describes balancing while charging batteries by using a transformer with two primary and secondary windings. These windings have the same number of turns.
U.S. Pat. No. 5,594,320 describes a balancing of the charge in the cells utilising a transformer containing the same number of windings as the number of cells.